The number of people dying from disease following the Indian Ocean tsunami could exceed those killed in the disaster itself, a health expert warns.
There is still no news from some areas affected by the disaster
Dr David Nabarro of the World Health Organisation said it was vital to get relief supplies to stricken areas to prevent a health catastrophe.
"There is certainly a chance we could have as many dying from communicable diseases as from the tsunami," he said.
Aid workers say providing clean water is their first priority.
It is feared diseases like malaria, dengue fever and cholera could easily spread in unsanitary conditions.
In India, scores of camps have been set up in affected areas. The aid agency Unicef said some cases of disease had already emerged.
"Getting clean water to people in the camps is critical at this point to head off the spread of disease," it said in a statement.
"We are closely monitoring the hygiene conditions... Some diarrhoeal cases have already been reported, so providing oral rehydration solution is critical," it said.
Hospitals in the region are urging people to boil water before drinking it.
But people are still using water from local wells, despite the contamination fears.
"Nobody told us not to drink this water. Nobody has told
us to boil the water," said Siddiqa, a mother of
four, in the Nagapattinam region.
Municipal authorities across the stricken region have been burying dead bodies even before they have been identified, fearing they will spread disease.
In Indonesia, the BBC's Rachel Harvey says human remains are being piled up under plastic sheeting prior to mass burial.
But Dr Alessandro Loretti of the World Health Organization said this could end up being a waste of resources, and that far greater priority should be given to the provision of clean water and the management of human waste.
"It will be living people who pollute the water," he told the BBC.
He said the WHO was sending enough water purification supplies and drugs for tens of thousands of people in Indonesia, the Maldives and Sri Lanka.
But he said even more urgent was the need to find out what was happening in parts of Indonesia and the Maldives yet to be reached.
Agencies say getting emergency supplies to the stricken areas in time to prevent disease will be very hard.
"It is going to be a huge problem getting relief even out of the airport" in Aceh, Indonesia, said Michael Enquist, the head of the United Nations Organisation for the Coordination of Humanitarian Affairs.
"There is no petrol, no food, no water and no vehicles
available," he told the AFP news agency.